Weld joint preparation
The preparation decides how much filler goes into the joint, whether the root is penetrated and whether the weld binds to the side wall. Below are the angle, root face and gap for every thickness to ISO 9692-1 — and what sits behind those numbers.
Short answer
Every edge preparation is described by four dimensions:
- s — thickness of the material being joined;
- α — included angle, the opening of the groove;
- c — root face, the unbevelled part of the edge at the root;
- b — root gap between the parts.
The choice comes down to two questions: what thickness and is there access from both sides. Up to 3–4 mm nothing is bevelled at all, up to 12 mm a single-V is enough, above that a double-V with two-sided access or a U with one-sided.
Thickness and preparation
| Thickness s, mm | Preparation | Angle α | Root face c, mm | Root gap b, mm | When |
|---|---|---|---|---|---|
| ≤ 3 … 4 | I — square edge | — | — | 0–3 | Thin material: the gap alone gives penetration |
| 3 … 12 | Single-V | 50–70° | 1–2 | 1–4 | The default when access is from one side only |
| 12 … 40 | Double-V (X) | 50–60° | 1–3 | 1–3 | Access from both sides: about half the deposit of a single-V |
| > 30 | Single-U | 8–12° | 1–3 | 1–3 | Heavy plate, one-sided access; needs machining |
| > 40 | Double-U | 8–12° | 1–3 | 1–3 | Very heavy plate, access from both sides |
| 5 … 20 | Single-bevel (½V) | 45–55° | 1–3 | 1–3 | T-joint with full penetration |
| > 15 | Double-bevel (K) | 45–55° | 1–3 | 1–3 | T-joint with full penetration, access from both sides |
| — | Fillet, no preparation | — | — | 0–2 | Where full penetration is not required |
The angle follows the process
The same plate gets a different angle for MMA and for MAG, and that is not a matter of habit:
| Process | Included angle α | Why |
|---|---|---|
| MMA (111) | 60° | A covered electrode needs room to enter the groove and for the slag to be chipped off |
| MAG (135) | 50–60° | The arc digs deeper, so the groove can be narrower — less deposit and less shrinkage |
| TIG (141) | 60–70° | The torch is wide and the pool has to stay visible; a narrow groove invites lack of fusion |
The difference looks small, but on 20 mm plate going from 60 to 50 degrees cuts the groove area by about a fifth — the same fifth off the filler, the gas and the time for every metre of weld.
ISO 9692-1 covers arc welding of steels — that is where the numbers above come from. -2 deals with submerged arc welding of steels, -3 with MIG and TIG welding of aluminium, -4 with clad products. Aluminium takes different angles from steel, so carrying these tables over to it is a mistake.
Why this is not a free choice
Too narrow a groove means lack of side-wall fusion (4011). The electrode cannot reach the root at the right angle, the arc grazes the flank, and the weld metal lies against it without joining. A planar defect, zero tolerance at levels B and C of ISO 5817 — and the hardest of all to catch on a radiograph.
Too wide a groove is not safer, only dearer. The area grows with the opening, so every extra ten degrees costs a good tenth more filler, gas and time. With the filler goes the heat input, and with the heat input the shrinkage and distortion: the more metal you put in, the further the structure moves as it cools.
Too large a root gap ends in burn-through (510) or a sagging root, too small a one in incomplete penetration (402): the arc has nothing to reach the root with. That is why the gap is set at tacking and held to a gauge, not judged by eye after grinding.
The root face is what saves the root from burning away. Without it the first run burns through the joint; with it the root melts under control. Too large a root face brings the opposite trouble: the root is never penetrated.
How the edge is prepared
The order never changes: cut — remove the dross and the overheated layer — clean — tack while holding the gap.
After flame and plasma cutting the edge carries a layer of oxides and a heat-affected zone; it is ground back to bright metal, otherwise the oxides end up in the pool and come out as inclusions (303). Plasma adds dross on the underside of the cut — that gets removed, not welded over.
Cleanliness applies not only to the groove itself but to 20–30 mm either side of it. Rust, paint, zinc, oil and moisture are sources of hydrogen and porosity: paint and oil give pores straight away, zinc adds highly toxic fume, and moisture off galvanised or wet plate remains the commonest cause of porosity in an unheated workshop.
U and double-U preparations are machined — milled or run on an edge bevelling machine. A torch cannot produce that profile, and trying to "approximate" it with a wide V eats the very saving the U was chosen for.
What next
The figures for your own thickness, process and access come from the joint preparation calculator — the same numbers as in the table above. How much filler and how many electrodes a joint prepared this way will swallow is worked out by the electrode consumption calculator. How the preparation is recorded on a drawing — welding symbols. What poor preparation turns into — lack of fusion and penetration.
Frequently asked questions
At what thickness do you start bevelling?
Around 3 mm for MMA and TIG, 4 mm for MAG — the MAG arc digs deeper and copes longer without a groove. Below that the root gap alone gives penetration, and a bevel only adds deposit and distortion.
Single-V or double-V on 20 mm plate?
If both sides are accessible, double-V. It takes roughly half the deposit of a single-V and halves the angular shrinkage, because the weld sits symmetrically about the mid-plane. With access from one side only you are left with the single-V, and on heavier plate the U — it removes even less metal, but the flank has to be machined.
What does too narrow a groove cost you?
Lack of side-wall fusion — imperfection 4011 to ISO 6520-1. The electrode or torch cannot reach the root at the right angle, the arc grazes the flank instead of melting it, and the weld metal lies against it without joining. It is a planar defect: zero tolerance at quality levels B and C, and the hardest of all to see on a radiograph.
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